Normal alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Shifted alphabet: VWXYZABCDEFGHIJKLMNOPQRSTU
That is what a Caesar Cipher is. I have encoded a message using my Caesar Cipher with a shift in the alphabet of 21.
Ocdn dn v nzxmzo hznnvbz.
The encrypted message above says: "This is a secret message."
I was also given a code to work out without the key. Code I was given: Aol gpwwf Dvtisl sprlk av lha Hwwslz. I used a Caesar cipher with a shift in the alphabet of 7. After the code was deciphered, it said: "The zippy womble liked to eat apples."
When the key is given, the person will easily be able to work out what the encrypted message says. This type of encryption is prone to a Brute Force attack which will easily work out which letter is which. The Brute Force attack which involves using every possible key to eventually find the correct key.
A Substitution Cipher is a method of encryption where the plaintext is replaced by the ciphertext and be anything from single letters, pairs of letters, triplets of letters and so on. When the Substitution Cipher is being unencrypted without the key, the person doing so does a inverse substitution.
A Frequency Analysis attack takes advantage of the most common letters in the English alphabet to find the key by assuming the most common letter used in a Substitution Cipher is 'e', and so on.